The present invention relates generally to cementitious compositions and certain forming methods to produce structural components therefrom in commercially significant quantities at acceptable rates of production throughout. The present invention relates more particularly to roofing tiles or the like extruded from a combination of very lightweight and lightweight aggregate, Portland cement composition, with or without fiber reinforcement and cured to possess a substantially reduced density without unacceptable loss of engineering strength or integrity. This invention is broadly applicable to other structural forms where weight savings are required and to other methods of manufacture where the economies realized through extrusion are not paramount.
Cement, and particularly Portland cement, has long been an engineering material of choice for a wide variety of structural components. Mixed with aggregate, such as sand and stone, concrete is probably the most commonly used building material ever employed. Concrete constituents of cement and aggregate are copious and relatively inexpensive. Their abundance is augmented by their ready availability throughout the world. These factors have contributed to the popularity of concrete as a standard building material. Consequently, perhaps no building compositions have been more studied or better understood than cement and concrete. Yet, despite considerable study at the theoretical level, actual production of building components is still largely more art than science.
The formability of concrete has led to its adaptability as a structural building material. Through the controlled addition of varying types and amounts of aggregate, concrete has been employed in literally hundreds of different shapes or forms in the building trades, from cast or block foundations to roofs, with scores of application in between. The present invention is primarily (but not exclusively) concerned with cement-based roofing materials, such as tiles or shakes, and therefore greater emphasis will be placed on these applications; though those skilled in the art will understand from the ensuing discussion how these principles are applicable to a host of other applications as well.
Despite the widespread popularity of cement as a building composition, its use as an effective roofing composition has been hampered and has not seen parallel adoption as, for example, when compared with foundations, walls or floors. There have been successful instances of its use for roofing tiles, especially in Europe, dating back as long as 100 years. However, those earlier attempts were based on manual production; each tile was essentially handmade for its specific application. Then too, those earlier efforts typically adopted cement compositions otherwise formulated for standard concrete forms; there was little or no consideration given to specific compositional variations to achieve weight savings.
The last 100 years have seen continuing interest in the use of cement as a structural composition for roofing materials. Many different suggestions have been made with this goal in mind. None has been entirely successful, in providing a cementitious roofing material having adequate strength which can be produced economically.
The prior art recognizes the need to reduce the weight of normal concrete mixes when forming roofing tiles or the like. The specific gravity of concrete is about 2.1-2.2. This is too heavy to be practical for many roofing applications. Thus, the art has adopted the use of lightweight aggregates to replace stone and sand conventionally used in concrete mixes. This substitution of materials allows producers to achieve densities in the range of about 1.6. Then, based on specific design configurations, roofing materials in the range of 6.5 to 7.0 pounds per square foot installed weight can be achieved. This is about 70% the weight of standard concrete roofing materials.
The use of lightweight aggregates alone cannot, however, yield a product having adequate engineering integrity without strict process controls. Usually these products are cast in forms or molds having the desired configuration for the finished tile. Unlike conventional concrete mixes, concrete with lightweight aggregate require dewatering and then curing under pressure to compact the aggregate and, thus, maintain its physical integrity during the curing process. Otherwise, two problems arise which diminish the strength of the finished product. As excess water evaporates during cure, voids remain which impair strength. Also, the very light aggregates, such as expanded Perlite, have a pronounced tendency to "spring back" after forming pressure removal and during cure, which not only can distort the product but will diminish cured strength as well. Curing under pressure eliminates these problem sources, all other variables being equal.
Casting lightweight aggregate cement and dewatering under pressure can lead to acceptable products in terms of strength and integrity. However, that process is slow and cumbersome. It is not suited to large scale production with minimal waste or defect rates. Conventional extrusion is better adapted for these purposes.
Concrete extrusion is a well known production technique. Though it works well when producing conventional concrete products using conventional concrete formulations, it has heretofore been impractical as a production technique for forming roofing tiles or the like from very lightweight aggregate. Except under laboratory or small scale production conditions, extrusion has proved too sensitive to variations in water content of the mix and the resultant product remains susceptible to spring back and attendant degradation in physical properties.
In consequence of the foregoing, it is apparent that there has been a long felt but unfulfilled need to develop a lightweight roofing product, having a specific gravity less than one half that of concrete. But the art has searched unsuccessfully for more than simply weight savings, the products heretofore elusive must also possess acceptable engineering strength and integrity. Moreover, products of this sort must be economical, which demands commercial scale production with acceptable rates of productivity (i.e., capable of modern manufacturing at the so-called "zero-defect" level).